Ubiquitous 60Hz electromagnetic noise generated by powerlines is often the dominant noise source in geophysical electromagnetic exploration. Powerline noise is frequently 100 to 1000 times larger in amplitude than the geophysical signal being measured, resulting in severe reductions in the effective dynamic range of the receiving equipment. The receiving equipment may have an actual dynamic range which is sufficiently large to record the signal of interest, but the large powerline noise signals prevent the use of the most sensitive end of the dynamic range because saturation of the amplifiers will occur. Thus, the dynamic range is effectively reduced by powerline noise.
In magnetotelluric exploration systems, natural electromagnetic signals are detected, which are usually much smaller than powerline noise. Reductions in dynamic range result in an inability to detect low-amplitude portions of the natural signals.
In controlled-source electromagnetic exploration systems, powerline noise reduces the sensitivity of the measurements. Controlled-source electromagnetic exploration systems ar either time domain (transient) systems or frequency domain systems. In transient systems, where a pulse of current through a transmitter antenna induces current into the earth and the decay of the induced current is measured, reductions in the dynamic range translates to reductions in the depth of penetration of the system. This is because the depth of penetration is dependent on the length of time the induced current signal can be measured before the signal degrades into noise. By reducing the level of noise, the length of time the induced current signal is measured is increased, resulting in an increased depth of penetration. In frequency domain systems, a continuous single frequency is transmitted from an antenna to induce current into the earth. The amplitude and phase of the induced current are measured with respect to the transmitted signal. For deep penetration, low frequencies are necessary, but at low frequencies, signal strength is small. Any source of detected noise will therefore reduce the dynamic range.
Prior art electromagnetic exploration systems receive the desired signal commingled with 60Hz noise. The 60Hz noise content is reduced by subsequent processing of the received signals. The success of reducing 60Hz noise by subsequent processing is limited in its stationariness, i.e., constancy of frequency, phase, and amplitude with time. The ability to record a small signal superimposed on the noise is limited by the dynamic range of the receiving equipment, and is directly related to the signal-to-noise ratio. The smaller the signal-to-noise ratio, the less success the processing will have in extracting the noise. Therefore, what is needed is a method and an apparatus that will reduce 60Hz noise before it reaches the receiving equipment, wherein the effective dynamic range of the receiving equipment can be increased.